1. Field of the Invention
The present invention relates to a distributed control system, and more particularly, to a system combined with a load sharing structure and a primary/backup structure for efficiently and stably operating the system.
2. Description of the Related Art
Generally, systems for distributively controlling many physically-distributed systems are divided into either a dual system (hereinafter, primary/backup structure) or a load sharing system. Systems that are primary/backup systems have resources and measures in place when a primary unit malfunctions. If a primary unit malfunctions in a primary/backup structure, the system can complete the processing of an event by using a backup unit. In such primary/backup systems, the backup unit contains all the information (is a duplicate) of the primary unit.
Systems that are load sharing have a plurality of units that can process events. The work in processing the events is distributed among the units so that to alleviate congestion that would result if only one unit is used.
However, primary/backup systems are inefficient because every piece of data must be duplicated into the backup unit. This requires a lot of overhead and resources to duplicate everything. Load sharing systems do not have primary/backup capabilities and systems with primary/backup capabilities do not have load sharing capabilities.
The above problems will be described as follows. The primary/backup structure does not have an efficient load sharing system and does not use all available system resources, thereby causing a waste of resources and deteriorating system efficiency. The primary/backup structure must be always increased in pairs, one for the primary unit and the other for the backup unit, causing an increase of cost.
The load sharing structure is advantageous in that it uses system resources efficiently by distributing processing of events in a uniform way among the units or sub-systems. However, this advantage is realized only when all systems are normally operated. If a certain system malfunctions in a load sharing structure, it becomes impossible to continue processing events in the down system, and furthermore, shared data occupied to be used cannot be restored. As a result, the load sharing system becomes unreliable because the load sharing system lacks backup and restoration capabilities.